1. Field of the Invention
This invention relates to ultrasonic Doppler diagnostic devices having a continuous wave (CW) Doppler mode, and in particular, to a method and apparatus for user selection of a Doppler sensitive portion of an imaged area and subsequent control of the CW Doppler mode for obtaining Doppler information from the user selected area.
2. Description of the Prior Art
The ultrasonic Doppler method is widely used for non-invasively detecting and measuring movement within a body, and finds wide use in medical ultrasound scanners for non-invasive diagnostic analysis of blood flow within a patient, e.g., for the detection and measurement of blood flow within the heart, blood vessels, etc., of a patient.
There are basically two operational modes of ultrasound Doppler; continuous wave (CW) and pulsed (PW). The PW mode is particularly useful for obtaining velocity data used to form a two-dimensional blood flow image (color flow image). However, because the pulse repetition rate (PRF) of PW Doppler systems limits the maximum flow velocity which can be determined without aliasing, the CW mode is found to be particularly useful for obtaining velocity data to accurately determine relatively high flow velocities.
Additionally, diagnostic ultrasound devices conventionally develop a so-called B-mode image, which is basically a two dimensional tomographic image, as well known to those skilled in the art. The B-mode image is formed using a transducer which can scan an area, and conventionally uses, as also well known in the art, one of several types of multiple element transducer arrays, such as a linear array of 64 or 128 transducer elements. On the other hand, CW Doppler operation generally requires the use of a special transducer probe which has two transducers, one for transmission and one for reception (as also well known in the art and commonly referred to as a pencil probe). It is desirable, for the sake of simplicity, to use a single probe and its receive signal processing circuitry to obtain the data useful for not only the Doppler mode, but also the B-mode. One technique to accomplish this is described in U.S. Pat. No. 4,915,115 entitled "Ultrasonic Imaging Apparatus for Displaying B-Mode and Doppler-Mode Images" and issued Apr. 10, 1990 to Sasaki et al. As described therein, data is obtained by first operating the transducer array via conventional transmit/receive control circuitry so as to provide steering and focusing of the transmitted ultrasound pulses and for providing appropriate individual delays to the received signals so that a conventional beamformer and image processor can create the B-mode image. Thereafter, the user can observe the B-mode image, and mark the image using a cursor with an indication of the beam direction and depth along that beam direction from which PW Doppler data is desired to be obtained. Then, the transducer is operated in the PW Doppler mode so as to receive Doppler signals therefrom, using range-gating techniques, and in turn develop Doppler data. It is noted that the CW Doppler mode is not described by Sasaki et al. Instead, Sasaki et al. is concerned with the reception efficiency of the transducer when it is desirable to operate it in both of the Doppler and B-mode, and solves this problem by providing an ultrasonic transducer having two peaks in its reception efficiency characteristic curves, one centered at the frequency used for B-mode imaging and the other centered at the frequency used for PW Doppler data gathering.
Using a multi-element transducer array for the CW Doppler mode is desirable because the ability to control transmit beam steering and focusing, as well as reception beam steering and focusing results in improved signal-to-noise (S/N) performance, as well as an opportunity to better select the Doppler sample volume (the overlap regions between the CW transmit and receive beams). U.S. Pat. No. 4,598,589 entitled "Method of CW Doppler Imaging Using Variably Focused Ultrasonic Transducer Array" and issued Jul. 8, 1986 to Riley et al. describes circuitry for operating such a multi-element transducer array so as to obtain improved S/N CW Doppler images, however, there is no indication about how a user of the apparatus would select or control the CW Doppler beam steering and/or focusing. The conventional techniques used for PW Doppler such as found in the forenoted U.S. Pat. No. 4,915,115 or in U.S. Pat. No. 4,416,286 entitled "Ultrasound Blood Flow Measuring Apparatus" issued Nov. 22, 1983 to Iinuma et al., would not be usable since CW Doppler does not use a range gate and therefore a selection of "depth" along a given receive beam line is inappropriate.
It is an object of the present invention to facilitate the use of a CW Doppler mode of operation for an ultrasound imaging apparatus which uses a multi-element transducer array.
It is a further object of the invention to provide a user control for selecting a CW Doppler sensitive area within a displayed two-dimensional ultrasound image.
It is a still further object of the invention to provide such a user control in a simple and low cost arrangement as well as in a way which can be easily controlled by the user.
These and other objects of the invention will become apparent from the following detailed description of a preferred embodiment of the invention, the accompanying drawings and the claims.